Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases. ... These changes result in a reduction of the metal's yield and tensile strength and an increase in its ductility, enabling further cold working.

What does cold working Increase?

Cold working increases the strength of a material, but decreases its ductility and electrical conductivity. In addition, residual stresses are introduced into the material because of the over-lapping and tangling of dislocations.

Why does cold work increase hardness?

When a metal is bent or shaped, dislocations are generated and move. As the number of dislocations in the crystal increases, they will get tangled or pinned and will not be able to move. This will strengthen the metal, making it harder to deform. This process is known as cold working.